As explained in U.S. Patent Publication 2007/0138563 (incorporated herein by reference) in complementary metal oxide semiconductor (CMOS) devices, polysilicon used to be the standard gate material. One advantage of using polysilicon gates is that they can sustain high temperatures. However, there are some problems associated with using a polysilicon gates and, therefore, metal gates are becoming more popular.
Further, as explained in U.S. Patent Publications 2005/0280104 and 2007/0141797 (incorporated herein by reference) the gate dielectric for metal oxide semiconductor field of fact transistor (MOSFET) devices has in the past typically comprised silicon dioxide, which has a dielectric constant of about 3.9. However, as devices are scaled down in size, using silicon dioxide as a gate dielectric material becomes a problem because of gate leakage current, which can degrade device performance. Therefore, there is a trend in the industry towards the development of the use of high dielectric constant (k) materials for use as the gate dielectric material of MOSFET devices. The term “high k material” as used herein refers to a dielectric material having a dielectric constant of about 4.0 or greater.
In dual metal gate transistor devices, metals of different work functions are used for the different types of transistors. Sometimes, an epitaxial layer can help maintain threshold voltage of transistors that utilize metal gates. However, epitaxially grown layers can have thickness inconsistencies, which results in inconsistent performance of the transistors.